1. Field of the Invention
The present invention relates to a control apparatus for a spark-ignition engine, and more particularly to a control apparatus that is suitable for achieving a high compression ratio of a direct injection-type spark-ignition engine and the miniaturization thereof.
2. Description of the Related Art
For improvement in the fuel consumption of a spark-ignition engine used for an automobile or the like, the achievement of a high compression ratio of the engine and the miniaturization (downsizing) of the engine are attracting increasing attention. The fuel consumption of engines can be improved by achieving a high compression ratio to enhance the theoretical thermal efficiency, and by combining supercharging with downsizing to reduce the pumping loss and friction. However, since these engines are subjected to high temperature and high pressure inside a combustion chamber during a compression-expansion stroke in comparison with usually used engines, undesired knocking would easily occur at the time of high-load operation.
Usually, in order to avoid knocking, the ignition timing is retarded (shifted to the retarded angle side) at the time of high-load operation so as to prevent the temperature and pressure inside a combustion chamber from becoming high. However, in this case, the difference between the optimum ignition timing at which the thermal efficiency is the highest and the actual ignition timing becomes large, which leads to a decrease in thermal efficiency. As a result, an effect of improving the fuel consumption, which is primarily produced by the high compression ratio and downsizing, cannot be achieved.
A first known technique for avoiding knocking at the time of high-load operation includes the steps of: providing a combustion chamber with a residual-gas sucking unit; and at the time of high-load operation, sucking a residual gas whose temperature is high so that the increase in compression end temperature inside the combustion chamber is suppressed, which makes it possible to avoid knocking without decreasing the thermal efficiency (for example, refer to JP-A-2006-77711).
A second known technique includes the steps of: when knocking is detected, injecting fuel in a plurality of parts (partially injecting fuel a plurality of times) in a direct injection (DI) engine; and carrying out at least one fuel injection in a compression stroke to make maximum use of an evaporative cooling effect produced by cylinder injection, which makes it possible to avoid knocking (for example, refer to JP-A-2006-329158).